1. Field of the Invention
The present invention relates to the dairy industry and, more particularly, to a process for producing kefir.
Kefir is a traditional foodstuff in a nutrient diet of various groups of population, including children, ill and weakened people. In the diet of this category of patients kefir accounts for up to 60% of the daily ration. From the standpoint of the modern concept of the nutrition hygiene the formula of this every-day product (kefir) has to be different. This, for baby's nutrition (from 6 months) it is necessary to use kefir with an increased content of protein as the main growth factor and with a lesser content of mineral salts and ethanol; for weakened persons, especially during the post-operation periods, a product is required which has a lowered content of lactose and an increased content of protein; for patients with gastro-intestinal tract diseases it is preferable to use a low-energy product possessing high antibacterial properties. A product with an increased content of protein may be used for nutrition of people after irradiation, since proteins contribute to a rapid removal of radioactive nuclides from the organism.
The therapeutic properties of kefir are caused by accumulation of the products of metabolism of microflora of the fungal leaven therein. The antibiotic substances produced by the kefir leaven microflora ensure antibiotic properties of the product which are manifested in respect of bacteria of the colibacillus group in the cultural group dilutions of 1:16, 1:32, Zonne chinelles 1:32, 1:64. During the storage of kefir a further accumulation of antibiotic substances occurs which contributes to improvement of its stability in storage.
2. Description of the Invention
Known in the art is a process for producing kefir, which comprises pasteurization of milk, its homogenization, cooling, leavening with leaven prepared with the use of kefir fungi, maturation and cooling of the final product (GB, B, 1300355). However, this process comprising the use of natural cows milk as the starting raw material does not guarantee the final product possessing the required organoleptic and rheological characteristics since, depending on the season of the year and geographic zones, conditions of feeding and variety of cattle the content of protein in milk can vary considerably. If the dry solids content in milk is less than 8.5%, it is impossible to obtain kefir with good rheological properties.
Known in the art is a process for producing a fermented-milk product (yoghurt) developed by "Unilever", a British company, which comprises concentrating the protein in the starting material to a mass portion thereof equal to 7-15% by way of ultrafiltration, thermal treatment of the concentrate at a temperature of 55.degree. to 65.degree. C. for 30 minutes or at 70.degree.-80.degree. C. without residence, cooling to a temperature of 40.degree.-50.degree. C., leavening with lactic-acid bacteria (of the species Lactobacillus bulgaricus, Streptococcus lactis, Streptococus thermophilis, Lactobacillus acidophilis), fermentation, packing and storage (cf. "Ultrafiltration Processing of Milk Raw Materials and Tendencies in its Further Treatment", Review Information, 1986, CNIITEMyasomolprom, Moscow, p. 23). The process of fermentation of yoghurt is based on lactic-acid fermentation which proceeds rather actively even upon increasing the protein concentration in milk up to 7-15% by mass. This is facilitated by rather high temperature (40.degree.-42.degree. C.) conditions of culturing and biochemical activity of the microorganisms incorporated in the leaven. The resulting product--yoghurt has an increased viscosity and a longer (as compared to kefir) storage period.
The cultures of microorganisms incorporated into the leaven for yoghurt form lactic acid which consists, for 90%, from the D(-) form which is non-physiological for a baby's organism, thus limiting the use of yoghurt for nutrition of certain categories of people.
The production of kefir is based on two, not one, processes, namely: the process of lactic-acid fermentation and the process of ethanolic fermentation.
The microflora of the fungal leaven for kefir incorporates not only lactic-acid bacteria, but yeast as well; the optimum for the development of the microorganisms included in the composition of the fungal kefir leaven is within the range of from 18.degree. to 26.degree. C. which differs substantially from that for yoghurt leaven--40.degree.-42.degree. C., wherefore the character of the development of these cultures in protein-enriched milk after ultrafiltration also differs substantially. Furthermore, upon elevation of the protein content in milk to over 7-15% by mass an adequate growth of the cultures incorporated in the kefir leaven (yeast) is not ensured. The fermentation process becomes extended in time, the product acquires a non-pronounced "empty" taste. Moreover, due to the long duration of the fermentation process in the final product proteolytic flaws appear: bitterness, foreign after-tastes.
Known in the art is a process for producing kefir which comprises enrichment of the desired product with a protein component. The process also involves purification of the starting milk, its normalization relative to the fat content, normalization relative to the dry solids content, heat-treatment, homogenization, cooling to the leavening temperature introduction of the leaven (prepared using kefir fungi) in a amount of 1 to 5% by mass of the milk, fermentation of resulting mixture, cooling of the curd, followed by maturation and cooling of the desired product. The normalization of milk relative to the content of dry solids is conducted by adding sodium caseinate to the fat-normalized milk. Dry sodium caseinate is preliminarily dissolved in milk at a temperature of from 60.degree. to 70.degree. C. (cf. Z. S. Zobkova et al. "Production of Milk and Dairy Products with Fillers and Vitamins", 1985, Agropromizdat Publishers, Moscow, p. 80).
However, the enrichment of kefir with dry sodium caseinate does not ensure the control of the desired product composition as regards the content of carbohydrates and mineral substances; neither does it improves the nutritions and biological value of the product and in many cases even lowers its organoleptic characteristics. This is due to the fact that in the preparation of sodium caseinate use is made of electrolytes and coagulation agents (CaCl.sub.2, HCl, polysaccharides) which cause phase and structure changes in the milk proteins that provides a detrimental effect on their biological and nutritious value. Furthermore, it becomes impossible to produce kefir possessing good organoleptic and rheological properties.